Influence of Parameterization Changes on Arctic Low Cloud Properties and Cloud Radiative Effects in Two Versions of the HadGEM3 Atmospheric Model: GA7.1 and GA6

Arctic clouds play a key role in Arctic climate variability and change; however, contemporary climate models struggle to simulate cloud properties accurately. Model-simulated cloud properties are determined by the physical parameterizations and their interactions within the model configuration. Quantifying effects of individual parameterization changes on model-simulated clouds informs efforts to improve models and provides insights on climate system behavior. This study quantities the influence of parameterization scheme changes on Arctic low cloud properties within the Hadley Center Global Environmental Model 3 atmospheric model using a suite of experiments where individual parameterization packages are changed between the two configurations. The results indicate, surprisingly, that single parameterization changes explain most of the cloud property changes, whereas multiple parameterizations, including non-cloud schemes, contribute to cloud radiative effect differences. Non-cloud parameterizations are those not used to compute time step cloud properties. We employ a three-term decomposition to quantify contributions from (a) regime independent, (b) regime dependent, and (c) the regime frequency of occurrence changes. Decomposition results indicate that cloud property changes vary by meteorological regime, each term contributes differently to each cloud property change, and non-cloud parameterization changes make substantial contributions to the LW and SW cloud radiative effects by affecting clear-sky fluxes. The analysis provides insights on the role of non-cloud parameterizations for setting cloud radiative effects, a pathway for cloud-atmosphere circulation interactions, raises questions on the importance of infrequently occurring regimes to climate simulations, and on useful observational approaches for improving models.